Check whether you already have a userDefineLang.xml. As mentioned on the SourceForge page, with my 64-bit Windows 7, rather than this being in C:\Program Files (x86)\Notepad++, it was required in <user>\AppData\Roaming\Notepad++.

If you do not have an userDefineLang.xml file already: save the file userDefineLang_VisualFoxPro.xml in this folder, then rename it to userDefineLang.xml.

If you do have an userDefineLang.xml file already: select all the text from userDefineLang_VisualFoxPro.xml, and paste it at the end of userDefineLang.xml. This will probably have created a spurious </Notepad_Plus><Notepad_plus> pair (or similar) in the middle, each tag on a line by itself. Remove these two consecutive lines.

Close Notepad++.

Run Notepad++.

Take menu option Languages. Near the bottom of the menu you should find an entry for “Visual FoxPro”. Syntax formatting should now work for this!